1. Field of the Invention
The present invention relates to a device and a method for separating a mixture of gases into product gas and off gas by means of gas permeation.
In gas permeation membrane units, the separation of product gas and off gas is conducted by means of permeation, wherein, for example, a product gas-enriched retentate and an off gas-enriched permeate can be obtained. The product gas concentrations in the retentate and the off gas concentrations in the permeate are, among other factors, dependent on the process parameters employed in each case; in general, an increased product gas quality will always require an increased use of energy (due to higher pressures, lower yields with respect to the feed gas used, and the like). Improved methods for increasing the product gas yield or for a more efficient utilization of energy in the course of such a method are thus desirable. Furthermore, it is desirable to keep investment costs for setting up a gas permeation plant as low as possible.
2. Description of Related Art
According to the state of the art, devices for separating a mixture of gases into product gas and off gas by means of gas permeation are designed such that the pressurized feed gas is separated into retentate and permeate in a membrane unit, wherein, for example, the retentate contains the product gas and the permeate contains the off gas. Disadvantages to this one-step solution are low product gas quality and low product gas yield, which entails an increased energy demand. Furthermore, an economical utilization of this device is only possible with the use of highly selective membranes.
Improved devices for separating a mixture of gases into product gas and off gas by means of gas permeation are designed such that the permeate of a first membrane unit is used in pressurized form as a feed gas for a second membrane unit, wherein the retentate flows of both membrane units contain the product gas and the permeate flow of the second membrane unit contains the off gas. Optionally, a compressor may be arranged upstream of the plant, unless the feed gas is present in a pressurized form. The advantage of this device is an improved product gas yield. Disadvantages to this solution are the still low product gas quality and an increased energy demand due to the required compaction of the gas for the second membrane unit. Furthermore, an economical utilization of this device is only possible with the use of highly selective membranes.
Furthermore, devices are known in which the retentate of a first membrane unit is used as a feed gas for a second membrane unit, the permeate of the second membrane unit is admixed to the pressurized feed gas of the first membrane unit, the retentate of the second membrane unit is withdrawn as a product gas and the permeate of the first membrane unit is withdrawn as an off gas. As in this case the permeate of the second membrane unit is circulated, in a manner of speaking, the dimensions of the plant and all required parts thereof (compressors, conduits, membrane units, cold separators, precision sulfur separators etc.) must be enlarged corresponding to the volume flow of the circulated permeate of the first membrane unit. Assuming a feed gas volume flow of 100 m3/h and the admixture of 80 m3/h of permeate of the second membrane unit to said feed gas will yield a total volume flow of 180 m3/h upstream of the compressor, according to which the plant is to be dimensioned. Advantageous to this method is the fact that a higher product gas yield can be obtained and that less selective membranes are used due to the two-step implementation; disadvantageous is the required oversized (by a factor of 1.2 to 2.5) dimensioning of the plant and the increased energy demand owing to recirculation.
Document U.S. Pat. No. 4,130,403 A (D1) discloses the back coupling of the retentate outlet of a membrane unit to the gas supply of a further membrane unit, which has only one single membrane unit arranged upstream thereof.
Agrawal R. et al. (Journal of Membrane Science, Elsevier Scientific Publ. Company, Vol. 112, No. 2) relates to cascading arrangements with two compressors. FIG. 6 shows back couplings to the gas inlet of the respective first membrane unit.
Document FR 2 917 305 A1 (D3) discloses a matrix arrangement of a plurality of membrane units.
Known from document WO 2010/141963 A1 is a device for separating a mixture of gases into product gas and off gas by means of gas permeation, said device having at least two membrane units (1) and (2) as well as a compressor (3) arranged upstream of the first membrane unit (1), wherein the membrane units (1) and (2) each have a gas inlet (1a, 2a), a retentate outlet (1b, 2b) and a permeate outlet (1c, 2c), wherein the retentate outlet (1b) of the first membrane unit (1) is connected via conduits to the gas inlet (2a) of the second membrane unit (2), the permeate outlet (2c) of the second membrane unit (2) is connected via conduits to the compressor (3) or the gas supply leading into the compressor on the suction side thereof, and the compressor (3) is connected via conduits to the gas inlet (1a) of the first membrane unit (1), product gas is obtained via the retentate outlet (2b) and off gas is obtained via the permeate outlet (1c).
According to WO 2010/141963 A1 it is provided in such a device that the permeate outlet (4c) of an upstream membrane unit (4) is connected via conduits to the gas supply of the compressor (3), wherein the membrane unit (4) has at least one further upstream membrane unit (5), which is formed by connecting the retentate outlet (5b) of said further membrane unit (5) to the gas inlet (4a) of the membrane unit (4) via conduits, and wherein additional product gas is obtained via the retentate outlet (4b) and additional off gas is obtained via the permeate outlet (5c).
All these devices have in common that the individual membrane units are always operated in counter-current flow.
Known from document WO 2010/141963 A1 are membrane units having a gas inlet, a retentate outlet and two permeate outlets, wherein the permeate spaces of such membrane units may be separated by a wall in the area located between both permeate outlets. Such membrane units are supposed to allow for an increased product gas yield.